Method and apparatus for controlling an amplifier

ABSTRACT

A method and apparatus are described for controlling the operation of an amplifier, in which, a bias level applied to the amplifier is adjusted so as to generate an output signal having the desired level of peak to mean ratio.

BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for controllingan amplifier according to an envelope ratio of a signal, and apparatusand methods for determining the peak-to-mean envelope ratio of a signal.In particular, the present invention may be applied to radio frequency(RF) signals such as used in the UMTS system.

RF transmissions from mobile telephones of the present generationtypically have a constant envelope shape, and the RF amplifiers operatewith constant power. However, in the next generation of mobiletelephones, it is intended to operate RF transmissions with a variableenvelope. It would be possible to continuously operate the RF amplifierat a high power, sufficient to generate the largest signal output. Forexample, it would be possible to operate an amplifier at a biasproviding a 1 W output capability despite a current output requirementof only 1 mW output power, in order to be capable of providing a 1 Woutput power when required. However, that would be wasteful of power, asa high standby current (bias) would need to be applied to the RFamplifier at all times. Mobile telephones are typically operated usingrechargeable batteries. It is required for the mobile telephone tooperate for as long as possible, both in standby and operating modes,from a single battery charge. Therefore, the power consumption of themobile telephone should be reduced as much as possible.

Some attempts have been made to adapt the bias (standby powerconsumption) according to the instantaneous value of the output signalenvelope. While this would reduce the power consumption of theamplifier, and extend battery life, it has not been found possible totrack rapid changes in envelope fast enough to allow efficient RFtransmission.

SUMMARY OF THE INVENTION

It is therefore required to provide a method and apparatus forefficiently determining and controlling the power consumption of anamplifier. In an RF system, the transmitter is likely to operate atrelatively low output power, due to the statistical distribution ofpower output in real networks. It is accordingly preferred that themethod and apparatus be particularly low in power consumption atrelatively low output powers.

According to an aspect of the present invention, a method forcontrolling the operation of an amplifier comprises the steps of:measuring a characteristic of an output signal of the amplifier;measuring a peak value of the characteristic; measuring a time-averagedmean value of the characteristic; comparing the peak value to the meanvalue; providing a demand signal representing a required value of thepeak to mean comparison; and adjusting a bias level applied to theamplifier so as to generate an output signal having the desired level ofthe peak to mean comparison.

According to an embodiment of the invention, a peak-to-mean enveloperatio of the signal is determined. The mean value is calculated over arelatively lengthy period of time, while peak values are detected andheld, again for a relatively lengthy period of time. Therefore, thepeak-to-mean value of the signal changes relatively slowly as comparedto the rate of change of the envelope itself.

The present invention therefore provides a peak to mean ratio detector;apparatus for controlling an amplifier, comprising such peak detector; amethod for controlling an amplifier; and a method for obtaining a peakenvelope to mean envelope ratio.

In particular, according to an aspect of the present invention, a methodfor controlling an amplifier comprises the steps of: detecting an outputpower envelope of a signal from the amplifier; deriving a first value,indicative of the output power envelope; and deriving second and thirdvalues, respectively indicative of a mean value, and a peak value, ofthe first value. The second value is subtracted from the third value, toobtain a difference value. A demand signal is supplied and compared tothe difference value, to derive a bias signal. The amplifier iscontrolled according to the bias signal, to achieve a desired peakenvelope to mean envelope ratio in the output signal of the amplifier.

In an embodiment of the present invention, the first value is alogarithmic representation of the output power envelope; the second andthird values are respectively mean and peak values of the logarithmicrepresentation of the output power envelope, and the demand signal isadapted to operate with the difference value to produce the requiredadjustment to the bias signal, to control the amplifier as required.

In an embodiment of the present invention, the first value is a linearrepresentation of the output power envelope; the second and third valuesare respectively linear representations of mean and peak values of thefirst value, the difference value is a linear representation of thedifference between peak and mean values of the output power envelope;and the demand signal is adapted to operate with the difference value toproduce the required adjustment to the bias signal, to control theamplifier as required.

In an embodiment of the present invention, the first value is a linearrepresentation of the output power envelope; the second and third valuesare respectively logarithmic representations of mean and peak values ofthe first value, the difference value is a logarithmic representation ofthe ratio of the peak value of the output power envelope to the meanvalue of the output power envelope; and the demand signal is adapted tooperate with the difference value to produce the required adjustment tothe bias signal, to control the amplifier as required.

In an embodiment of the present invention, the first value is anarbitrary polynomial representation of the output power envelope; thesecond and third values are respectively mean, and peak, values of thefirst value, and the demand signal is adapted to operate with thedifference value to produce the required adjustment to the bias signal,to control the amplifier as required.

The third value may be obtained by passing a signal representing thefirst value in succession through a low pass filter and a peak detector.

The second value may be obtained by applying a signal representing thefirst value through an integrator.

The step of detecting an output power envelope of the signal from theamplifier may comprise the step of downconversion prior to detection.

According to a further aspect of the present invention, apparatus forcontrolling an amplifier to achieve a desired peak envelope to meanenvelope ratio in the output signal of the amplifier, comprises adetector for deriving a first value, representative of an output powerenvelope of a signal from the amplifier; a peak detector for detecting apeak value of the first value; an averaging means for detecting a meanvalue of the first value. A first subtracter is provided for subtractingthe mean value from the peak value, to obtain a difference value. Asecond subtracter is provided for subtracting the difference value froma demand signal to produce a bias signal. An input to the amplifier isprovided for receiving the bias signal, to control the amplifieraccording to the difference between the difference value and the demandsignal, so as to achieve a required peak-to-mean ratio of the outputsignal.

The detector may be a logarithmic detector, the first value may be alogarithmic representation of the output power envelope, and the demandsignal may be adapted to operate with the difference value to producethe required adjustment to the bias signal, to control the amplifier asrequired.

Alternatively, the detector may be a linear detector, the first valuemay be a linear representation of the output power envelope, and thedemand signal may be adapted to operate with the difference value toproduce the required adjustment to the bias signal, to control theamplifier as required. In this case, the difference value may representthe difference between the peak value of the output power envelope andthe mean value of the output power envelope.

Alternatively, the detector may be a linear detector and the first valuemay be a linear representation of the output power envelope. Logarithmicconverters may be provided for converting the peak value and the meanvalue into logarithmic representations. In this case, the demand signalis adapted to operate with the difference value to produce the requiredadjustment to the bias signal, to control the amplifier as required. Thedifference value will represent the logarithm of the ratio between thepeak value of the output power envelope and the mean value of the outputpower envelope.

Alternatively, the detector may be an arbitrary polynomial detector, andthe first value may be a corresponding arbitrary polynomialrepresentation of the output power envelope. In this case, the demandsignal will be adapted to operate with the difference value to producethe required adjustment to the bias signal, to control the amplifier asrequired.

In any such apparatus, the peak value may be obtained by passing asignal representing the first value in succession through a low passfilter and a peak detector. The mean value may be obtained by applying asignal representing the first value through an integrator.

A downconverter may additionally be provided for downconverting thesignal prior to detection of the output power envelope.

In certain embodiments of the invention, the amplifier is an RFamplifier.

According to a further aspect of the present invention, a method forobtaining a peak envelope to mean envelope ratio measurement of a firstsignal comprises the steps of: deriving a second signal indicative ofthe envelope of the first signal; deriving a third signal, indicative ofthe mean of the second signal; deriving a fourth signal, indicative of apeak value of the second signal; and subtracting the third signal fromthe fourth signal, to derive a fifth signal, indicative of the peakenvelope to mean envelope ratio of the signal.

In such a method, the second signal may represent a logarithm of theenvelope of the first signal, and the fifth signal may represent alogarithm of the peak envelope to mean envelope ratio of the firstsignal.

The signals may be transmitted as electrical or optical signals. Thesignals may be transmitted as analogue levels, or may be digitallyencoded, or may be pulse-modulated waveforms.

The above, and further, objects, characteristics and advantages of thepresent invention will become more apparent with reference to thefollowing description of certain embodiments thereof, given by way ofexamples only, in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A represents an input power to output power relationship for atypical linear amplifier;

FIG. 1B represents a typical envelope waveform; and

FIG. 2 represents apparatus for controlling an amplifier according tothe present invention, incorporating a peak to mean detector accordingto the present invention, operable according to the method of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In some radio frequency systems, such as the UMTS system, it isnecessary to provide a signal with a desired peak-to-mean enveloperatio. In order to achieve a higher data rate from a given bandwidth ofthe RF spectrum, it is necessary to employ a signal with a non-constantenvelope, giving rise to a high peak-to-mean ratio.

In order to generate RF signals with a varying envelope, linear poweramplifiers are required, and these may be set up to provide some levelof clipping to the output signal. Allowing some degree of clippingallows the amplifier to operate at lower power consumption, givingincreased battery life.

Linear power amplifiers are known, having a bias input, which sets thebias point, which in turn determines the current consumption and so alsothe maximum output power. A low bias leads to a low peak output power.Gain is also affected by bias but less strongly, thus affecting the meanoutput power.

Linear amplifiers are only linear up to a compression point, which isaffected by the bias. FIG. 1A shows the relationship between outputpower Pout and input power Pin for a linear amplifier with various biaspoints. Curve 101 illustrates the output power to input powerrelationship for a linear amplifier operated at a relatively high bias,while the curve 102 illustrates a corresponding relationship for thesame amplifier operated at a lower bias. Mean input power Pim and meanoutput power Pom values are shown, along with a peak input power Pip,and corresponding peak output values Pop, Pop′ for low and high biascases, respectively. In the low bias case 102, the peak-to-mean ratio(Pop/Pom) is reduced by compression in the amplifier. The high biaspoint 101 should then be chosen to provide a higher peak-to-mean ratio(Pop′/Pom), and to preserve the integrity of the input signal. However,if the magnitude of the input signal (Pim, and particularly Pip) isreduced, the lower bias 102 would suffice. Current would be wasted ifthe high bias 101 is chosen to cope with input signels of greatermagnitude than necessary.

A required peak-to-mean ratio should be chosen such that the resultantsignal may be clipped, sufficiently to achieve maximum efficiency, whilenot being so far clipped that spectral spillage limits are exceeded. Byselecting a peak to mean ratio, the amplifier bias may be set to providejust enough capability to amplify an input signal to an output signalwith a certain level of clipping, that is, to operate at an optimalpower consumption whatever the level of input signal.

According to an aspect of the present invention, a peak-to-mean value ofthe envelope of the output signal is measured, using a mean valueobtained over a relatively long period of time, while the peak valuesare detected and held over a relatively long period of time. Therefore,the peak-to-mean values determined according to the methods andapparatus of the present invention vary relatively slowly. The methodsand apparatus of the present invention do not attempt to vary the biaslevel of the amplifier according to real-time variations of the RFsignal.

It is therefore possible that the amplifier bias is excessive for atime, following an abrupt reduction in envelope size, or the envelopebias may be undervalued for a time, following an abrupt increase inenvelope size. However, the operation has been found to be satisfactory,with significant reductions in power consumption being achieved. Theapparatus and method of the present invention may provide for resettingof variables if an abrupt change is to be made to the level of the inputsignal to the amplifier.

The present invention particularly relates to an efficient way ofdetermining the peak-to-mean ratio of a signal envelope, and anamplifier controller using the peak-to-mean ratio detector.

The peak to mean detector of the present invention is a system conceptthat allows the bias of a linear power amplifier to be set to produce ademanded peak to mean ratio of the signal at the output of theamplifier. This allows the overall system to minimise its powerconsumption. The sought peak-to-mean ratio control is a long-termcontrol servo that varies the bias based on the history of the outputsignal. It does not aim to control at the modulation rate, and istherefore equally applicable to narrowband and wideband modulationschemes.

The most important part of the apparatus of the present invention is thepeak-to-mean ratio detector. This is a circuit which can estimate thepeak and mean envelopes of the output signal, and generate a ratioedoutput that can act as a measurement of the peak to mean value of theoutput signal which is used to control the amplifier.

As is widely recognized, the operation of mathematical division isdifficult in electronic or other control circuits, whereas processes ofaddition and subtraction are relatively easy. Therefore, according toone aspect of the present invention, there is provided a method forobtaining the peak-to-mean ratio without performing a divisionoperation.

According to certain embodiments of the present invention, themathematical operation that logarithm of the ratio of two quantities canbe expressed as a subtraction of the logarithms of the two quantities,is used:log_(x)(A/B)=log_(x)(A)−log_(x)(B).

Therefore, the logarithm of the peak-to-mean envelope is equal to thedifference between the logarithm of the peak value of the envelope andthe logarithm of the mean value of the envelope.

Referring now to FIG. 2 of the drawings, a linear power amplifier 10 isto be set by a bias signal 16, determined according to an input signal12 representing a demanded peak to mean ratio, to be applied to theoutput signal 14. The demanded ratio is determined by consideration ofthe level of clipping that is acceptable. This is easily simulated ormeasured in the design of the apparatus and should be only a function ofthe number of channels being generated by the transmitter. There isalways one pilot channel and one data/voice channel in UMTS as aminimum. If more data channels are added, the peak/mean increases and adifferent demanded peak-to-mean ratio is needed. The criterion for theoutput peak-to-mean ratio is that the output signal meets the relevantspectral requirements and also the “vector error magnitude” ormodulation accuracy requirements. A sampling device 15, such as acoupler, directional coupler or resistive tap, diverts some of theoutput signal energy from the amplifier into a peak-to-mean detector 20of the present invention. A downconverter 22 may be provided, dependingon the frequency of the output signal 14, to convert the signal 14 intoa signal 24 of more appropriate frequency.

The downconverter 22, where provided, may comprise an isolationamplifier 22 a, supplying a signal to a mixer 22 b. The mixer 22 b mayalso receive a local oscillator signal 22 c from local oscillator 22 dto provide a mixed signal 22 e, which is then passed through a band passfilter 22 f of the required frequency band.

The output of the downconverter, or the output of the amplifier, if ofsuitable frequency, is then applied to a logamp detector 26. Thisprovides a signal 28 representative of the logarithm of the envelope ofthe signal 14. Suitable logamp detectors are known, for example, as usedin spectrum analysers and radar receivers.

The signal 28 is then applied to a peak detecting means 30 and to anaveraging means 32, in parallel. The averaging means includes anintegrator 34, which receives the signal 28 and outputs a value 40representing the mean value of the logarithm of the envelope of thesignal 14. The peak detecting means typically comprises a low passfilter 36 and a peak detector 38, connected in series in that order.

FIG. 1B illustrates an example in the value of the envelope e with timet. Curve 103 illustrates a typical variation of envelope with time, andpeak values can be seen, while a mean value could be estimated. In theenvelope waveform 103, so-called “super peaks” 104 may occur, ie peakenvelope values much higher than the normal peak envelope values. Theseare comparatively rare, have a very fast rise time and are narrow.Because they are rare, it is perfectly possible and usually acceptableto clip them, without degrading performance. The low pass filter 36serves to filter off the size of these narrow peaks (clip the superpeaks), thereby preventing the overloading of the peak detector 38.

As a subsidiary function, the low pass filter is preferably also adaptedto eliminate high frequency, e.g. RF, components, to pass only theenvelope to peak detector 38

The peak detector 38 then detects a maximum value of the envelope of theinput signal 28, and holds that value as its output signal 42. The peakdetector holds this value for a relatively lengthy period of time. Thedecay rate of the peak detector should be as slow as possible, whilestill providing detection of the majority of peaks within the envelopeof signal 14. In a typical RF signal for mobile telephony applications,peak values may last only 10 ns or so, while the decay rate should beset slow enough to retain the peak value for several milliseconds, asimilar time to that used for averaging in the mean detector 34.

The signal 40 (representing the mean value of the logarithm of theenvelope of the signal 14), and the signal 42 (representing the maximumvalue of the logarithm of the envelope of the signal 14) are thenapplied to a subtracter 44, which subtracts the value of signal 40 fromthe value of signal 42, and provides a difference signal 46,representing the logarithm of the peak to mean value of the envelope ofsignal 14.

A second subtracter 48 receives the difference signal 46 and subtractsit from the demand signal 12, which it also receives. The differencebetween the two signals is output as an error signal 50. The errorsignal 50 is preferably, although not necessarily, then applied to asmoothing circuit (integrator 52), before being used as the bias controlsignal 16. The integrator 52 is there to define the controlcharacteristics, thereby assuring that the loop settles with zero errorwith a non-overshooting response.

In the above described embodiment, a logarithmic detector 26 is used,that is, the envelope value is detected, and its logarithm derived atthe same time, to directly produce an output signal 28 which isrepresentative of the logarithm of the envelope. The output 46 ofsubtracter 44 therefore represents the difference between the peak ofthe logarithm of the envelope, and the mean of the logarithm of theenvelope. The mean of the logarithm of the envelope is slightlydifferent from the logarithm of the mean of the envelope, and so thevalue of the signal 46 will not exactly represent the logarithm of themean of the envelope. However, this difference may be compensated for byvarying the values of the demand signal 12 applied to request a certainpeak-to-mean ratio.

In alternative embodiments, the envelope detection function may beseparated from the logarithm derivation. For example, a diode detectormay be used to provide a linear indication of the envelope, with alogarithmic converter applied later. The logarithmic converter may beplaced between the diode detector and the peak and mean detectors.Individual logarithmic converters may be provided for each detector, ora single logarithmic converter may provide a single output that isdirected to both peak and mean detectors. In further alternativeembodiments, individual logarithmic converters may be placed after eachof the peak and mean detectors, to convert linear values of peak andmean into logarithmic values. In these embodiments, the signal 46produced by the subtracter 44 will represent the actual logarithm of thepeak-to-mean ratio.

In yet further embodiments of the present invention, logarithmicconversion may be avoided altogether. For example, diode conversion maybe used directly with a peak detector and a mean detector, and peak andmean envelope values applied to the subtracter. The signal 46 suppliedby the subtracter will no longer represent the peak to mean ratio, butwill rather represent a difference between peak value and mean value.

The principles of the present invention apply whether the differencesignal 46 produced by the subtracter represents a logarithm of the peakto mean ratio, or a difference of the logarithms of the peak and meanvalues, or a difference of some other function. The demand signal 12applied to second subtracter 48 must be adapted to correspond to thetype of difference signal in use.

For example, the demand signal 12 may represent the logarithm of therequired peak-to-mean envelope, to allow a direct comparison with thedifference signal 46. Alternatively, a linear demand signal may beprovided to a logarithmic converter (not shown) before being applied 12to the subtracter 48. A further alternative would be to convert thedifference signal 46 into a linear signal before applying it to thesubtracter 48, and using a linear demand signal 12.

Some logarithmic converters, such as the logamps referred to earlier,may be difficult to stabilise in temperature. When used to derive theenvelope, temperature stabilisation is not important as differentialpairs are used, and are relatively immune to temperature variation.However, when used as a standalone logarithmic converter, temperaturestabilisation becomes important.

In a second, alternative, embodiment the peak detection means 30 and theaveraging means 32 each include a logamp detector, similar to thatindicated at 26, placed upstream of the components illustrated in thedrawing. Logamp converter 26 would not then be required.

In a third embodiment, the logamp detectors referred to in respect ofthe second embodiment are located downstream of the respectivecomponents illustrated in the drawing. In this embodiment, thesubtracter is supplied with the logarithm of the mean of the envelope,and the logarithm of the peak of the envelope, whereas in the otherembodiments, the subtracter is supplied with the mean of the logarithmof the envelope, and the peak of the logarithm of the envelope. In suchan embodiment, the value of the demand signal 12 has to be chosencorrectly. It should be related to the logarithm of the differencerather than the simple difference of the logarithms. This is still justa simple voltage. There is a slight increase in sensitivity to thedemand input in this case, since the error voltage is smaller.

While the present invention has been described with reference to alimited number of particular embodiments, various modifications andadjustments may be made within the scope of the present invention. Forexample, although the present invention is particularly applicable to RFsignals, it may be applied without substantial modification to signalsof other wavelengths.

The preceding description refers only to “logarithms”, the presentinvention will perform equally well using logarithms to base 10, base e,base 2, base 16 or any other base. For this reason, the formula recitedabove is expressed in logarithms to base x. As discussed earlier, thepresent invention may also be applied to arrangements not having alogarithmic function.

The signals referred to above may be embodied as electrical, optical,mechanical or other suitable signals in a circuit designed to respond tosuch signals. The signals may be expressed as an analogue value (e.g. asteady voltage, a steady light intensity, a steady pressure), or by adigital coding of some sort (e.g. hexadecimal numbering) or a pulse codesystem (e.g. where the value expressed is related to the instantaneousfrequency of digital impulses). It may be necessary to convert betweentypes of signal for processing (e.g. optical or mechanical signals 14may need to be converted into electrical signals before they can beprocessed).

The forgoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

1. A method for controlling the operation of an amplifier, comprisingthe steps of: measuring a characteristic of an output signal of theamplifier; measuring a peak value of the characteristic; measuring atime-averaged mean value of the characteristic; comparing the peak valueto the mean value; providing a demand signal representing a requiredvalue of the peak to mean comparison; and adjusting a bias level appliedto the amplifier so as to generate an output signal having the desiredlevel of the peak to mean comparison.
 2. A method for controlling anamplifier (10) according to claim 1, comprising the steps of: detectingan output power envelope of a signal from the amplifier; deriving afirst value, indicative of the output power envelope; deriving secondand third values, respectively indicative of a mean value, and a peakvalue, of the first value; subtracting the second value from the thirdvalue, to obtain a difference value; supplying a demand signal andcomparing it to the difference value, to derive a bias signal; andcontrolling the amplifier according to the bias signal, to achieve adesired peak envelope to mean envelope ratio in the output signal of theamplifier.
 3. A method according to claim 2, wherein the first value isa logarithmic representation of the output power envelope; the secondand third values are respectively mean and peak values of thelogarithmic representation of the output power envelope, and the demandsignal is adapted to operate with the difference value to produce therequired adjustment to the bias signal, to control the amplifier asrequired.
 4. A method according to claim 2, wherein the first value is alinear representation of the output power envelope; the second and thirdvalues are respectively linear representations of mean and peak valuesof the first value, the difference value is a linear representation ofthe difference between peak and mean values of the output powerenvelope; and the demand signal is adapted to operate with thedifference value to produce the required adjustment to the bias signal,to control the amplifier as required.
 5. A method according to claim 2,wherein the first value is a linear representation of the output powerenvelope; the second and third values are respectively logarithmicrepresentations of mean and peak values of the first value, thedifference value is a logarithmic representation of the ratio of thepeak value of the output power envelope to the mean value of the outputpower envelope; and the demand signal is adapted to operate with thedifference value to produce the required adjustment to the bias signal,to control the amplifier as required.
 6. A method according to claim 2,wherein the first value is an arbitrary polynomial representation of theoutput power envelope; the second and third values are respectivelymean, and peak, values of the first value, and the demand signal isadapted to operate with the difference value to produce the requiredadjustment to the bias signal, to control the amplifier as required. 7.A method according to claim 2, wherein the third value is obtained bypassing a signal representing the first value in succession through alow pass filter and a peak detector.
 8. A method according to claim 2,wherein the second value is obtained by applying a signal representingthe first value through an integrator.
 9. A method according to claim 2,wherein the step of detecting an output power envelope of the signalfrom the amplifier comprises the step of downconversion prior todetection.
 10. A method or Apparatus according to claim 1, wherein theamplifier is an RF amplifier.
 11. A method or apparatus according toclaim 1, wherein the signals are transmitted as electrical or opticalsignals.
 12. A method or an apparatus according to claim 1, wherein thesignals are transmitted as analogue levels, or are digitally encoded, orare pulse modulated waveforms.
 13. Apparatus for controlling anamplifier to achieve a desired peak envelope to mean envelope ratio inthe output signal of such amplifier comprising: a detector generatingfor a first value, representative of an output power envelope of asignal from such amplifier; a peak detector for determining a peak valueof the first value; an averaging means for determining a mean value ofthe first value; a subtracter for subtracting the mean value from thepeak value, to obtain a difference value; a subtracter for subtractingthe difference value from a demand signal to produce a bias signal forcontrolling such amplifier according to the difference between thedifference value and the demand signal, so as to achieve a requiredpeak-to-mean ratio of the output signal.
 14. Apparatus according toclaim 13, wherein the detector is a logarithmic detector, the firstvalue is a logarithmic representation of the output power envelope, andthe demand signal is adapted to operate with the difference value toproduce the required adjustment to the bias signal, to control theamplifier as required.
 15. Apparatus according 13, wherein the detectoris a linear detector, the first value is a linear representation of theoutput power envelope, and the demand signal is adapted to operate withthe difference value to produce the required adjustment to the biassignal, to control the amplifier as required, wherein the differencevalue represents the difference between the peak value of the outputpower envelope and the mean value of the output power envelope. 16.Apparatus according to claim 13, wherein the detector is a lineardetector and the first value is a linear representation of the outputpower envelope, and wherein logarithmic converters are provided forconverting the peak value and the mean value into logarithmicrepresentations and wherein the demand signal is adapted to operate withthe difference value to produce the required adjustment to the biassignal, to control the amplifier as required, wherein the differencevalue represents the logarithm of the ratio between the peak value ofthe output power envelope and the mean value of the output powerenvelope.
 17. Apparatus according to claim 13, wherein the detector isan arbitrary polynomial detector, the first value is a correspondingarbitrary polynomial representation of the output power envelope, andthe demand signal is adapted to operate with the difference value toproduce the required adjustment to the bias signal, to control theamplifier as required.
 18. Apparatus according to claim 13, wherein thepeak value is obtained by passing a signal representing the first valuein succession through a low pass filter and a peak detector. 19.Apparatus according to claim 17, wherein the mean value is obtained byapplying a signal representing the first value through an integrator.20. Apparatus according to claim 13, wherein a downconverter is providedfor downconverting the signal prior to detection of the output powerenvelope.